LONDON — Britain’s prime minister on Tuesday ordered an investigation into cladding and insulation on high-rise towers across the country, as the authorities struggled to contain the risk from construction materials that leave tall structures vulnerable to catastrophic fires.

The government announced that 120 high-rise buildings, in 37 areas, had used cladding and insulation similar to those installed at Grenfell Tower, the West London apartment building that was consumed by flames on June 14 in Britain’s deadliest fire in decades. All 120 buildings fell short in safety tests — “a 100 percent failure rate,” according to a spokesman for Prime Minister Theresa May. Hundreds of other tall buildings are still being tested.

The cladding on the buildings will almost certainly have to be replaced. Such work has already begun on a complex of towers in the Salford area near Manchester. Last week, the London borough of Camden ordered the evacuation of 4,000 residents of a five-tower complex, Chalcots Estate, where the buildings were covered in the same flammable cladding used on Grenfell Tower.

Anxiety about the building materials appeared to be spreading to other countries, even though the United States and most countries in Europe had restrictions on the kind of cladding used on Grenfell Tower. The German city of Wuppertal on Tuesday evacuated about 80 people from an 11-story apartment tower “for fire safety reasons” after “flammable material was found,” a local official said.

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Isaac PaulousCreditMetropolitan Police

Arconic, the American company that sold the combustible material used at the London housing project, said on Monday that it would no longer sell the paneling for use in high rises. On Tuesday, the company’s stock dropped 9 percent on the New York Stock Exchange, and is down 21 percent since the fire.

Whirlpool, which in 2014 acquired the manufacturer of the refrigerator that started the fire, has urged owners of the fridge model to check their units.

The human toll of the fire, which killed at least 79 people, continued to be felt on Tuesday, as the authorities identified a 5-year-old boy, the youngest victim known so far.

The boy, Isaac Paulous, who was identified from his dental records, died from smoke inhalation, the inquest at Westminster Coroner’s Court found. He is among 19 victims to have been formally identified in the fire. The boy choked to death as he and his family tried to escape the flames in their 18th-floor apartment.

Isaac’s family described him as “kind,” “energetic” and “generous.”

“He was such a good boy, who was loved by his friends and family,” they said in a written statement. “We will miss him forever, but we know God is looking after him now and that he is safe in heaven.”

His mother, Genet Shawo, told The Times of London that her hand had slipped from her son’s as neighbors tried to lead the family to safety.

Among other newly identified victims were Mohamednur Tuccu, 44, whose body was recovered outside the building, close to a sports center; Khadija Saye, 24, and her mother, Mary Mendy, 52, who died from “inhalation of fire fumes and burns,” according to a preliminary report.

“It can’t really be explained in enough detail how complex an investigation this is in terms of identification, and in terms of recovery of bodies on a dangerous site that my team are not allowed to enter because the building is being shored up,” Dr. Fiona Wilcox, the coroner who conducted the inquests, said in a statement on Monday.

The blaze that engulfed Grenfell Tower on June 14 started with a faulty fridge on the fourth floor and spread to the top of the building within minutes. British police officers said the building’s insulation and exterior cladding had failed safety tests conducted by investigators after the fire.

Residents of the building, who had warned for years about fire hazards there, have said the disaster could have been prevented, and they blame the local council for the Royal Borough of Kensington and Chelsea, which owns the building, for using cheap, flammable materials during a renovation completed last year that cost 10 million pounds, or $12.8 million. The council’s chief executive was forced out last week.

A version of this article appears in print on , on Page A6 of the New York edition with the headline: British Leader Orders Investigation Into Cladding After Deadly High-Rise Fire. Order Reprints | Today’s Paper | Subscribe